The present invention relates to optical communication systems and more particularly to systems and methods for amplifying wavelength-division-multiplexed (WDM) signals.
The explosion of communication services, ranging from video teleconferencing to electronic commerce has spawned a new era of personal and business interactions. As evident in the rapid growth of internet traffic, consumers and businesses have embraced broadband services, viewing them as a necessity. However, this enormous growth in traffic challenges the telecommunication industry to develop technology that will greatly expand the bandwidth of communication networks. Further improvements in optical communications hold great promise to meet the demand for greater bandwidth.
Wavelength division multiplexing (WDM) technology permits the concurrent transmission of multiple channels over a common optical fiber, thus expanding available bandwidth and providing other advantages in implementation. In a WDM link between two points, it may be necessary to amplify the WDM signal at various locations. For example, amplification may be required at the transmitter, the receiver, or at intermediate points along the link.
It is desirable in certain situations to amplify and control power of sub-bands of the WDM signal independently. To assure effective automatic power control operation on each of the sub-bands, it is important that there be low crosstalk between the sub-bands. Furthermore, to assure optimal communication performance, it is desirable that the optical amplification system exhibit a low noise figure, i.e., output signal to noise ratio over input signal to noise ratio. It is further desirable that loss of amplifier input be independently detected for each sub-band so that automatic shutdown safety features may be correctly implemented. Other desirable features include ease of expandability to accommodate adding extra groups of WDM channels to existing systems and minimum volume consumption for the amplification system packaging.
FIG. 1 depicts an optical amplification system 100 according to one prior art approach. The design of FIG. 1 is intended for a C-band WDM system where there are multiple WDM channels to be amplified. This C-band is divided into two sub-bands, which require separate power control: a xe2x80x9credxe2x80x9d band (1540-1560 nm) and a xe2x80x9cbluexe2x80x9d band (1529-1535 nm). Both the red and blue bands are amplified within a first amplification stage 102 that is pumped in a co-propagating mode by a pump laser 104. The red band is separated from the blue band by a band separator 106 and then amplified within a second amplification stage 108. Second amplification stage 108 is also pumped by pump laser 104 but in a counter-propagating mode.
Monitoring of the input signal is achieved by photodiode 110. Monitoring of the blue band and red band amplified outputs is achieved by photodiodes 112 and 114 respectively.
Other components of optical amplification system 100 are included to appropriately filter and direct the various optical signals. A wavelength selective filter 116 separates the LSM (Line Service Modem) signals at 1480 nm and 1510 nm from the C band signals. The LSM signals carry various telemetry information. A tap coupler 118 removes a small portion of the C band signal for monitoring by photodiode 110. An isolator 120 suppresses undesired oscillation within first amplification stage 102.
A wavelength-selective filter 122 combines the pump laser energy with the C-band signal for input to the active fiber that implements first amplification stage 102. A tap coupler 124 taps off a small portion of the output of pump laser 104 for monitoring by photodiode 126. A wavelength-selective filter 128 separates the pump energy from the amplified C band signal.
The C band signal passes through an isolator 130 into band separator 106 while the pump energy is coupled into the active fiber implementing second optical amplification stage 108 by a wavelength-selective filter 132. A small portion of the blue band signal is tapped off by a coupler 134. A splitter 136 divides the blue band monitor signal between photodiode 112 and a monitor output. Similarly for the red band, a tap coupler 138 separates out of a portion of the red band signal for monitoring purposes. The red band monitor signal is split by a splitter 140 into one component that is input to photodiode 114 and another component that is presented at a red band monitor output.
This design fails to achieve many of the objectives given above for a WDM optical amplification system. The only way to regulate output power for either the red band or the blue band is by controlling pump current to pump laser 104. However, reducing the output of pump laser 104 to control red band output power will also have the effect of reducing blue band output power since pump laser 104 is also the pump energy source for first amplification stage 102 that amplifies both bands. Furthermore, if pump laser 104 is adjusted to regulate the blue band output power not only will red band output power be affected but also red band noise figure will be changed because of the reduction of gain in the first stage. Another obstacle to correct automatic power control is that band separator 106 is insufficient to provide good isolation between the red band and blue band signals so that in fact the blue path incorporates some red band energy or vice versa due to the limitations of filter technology. Thus, blue band power regulation will be based in part on extraneous red band energy and vice versa.
Another drawback is that photodiode 110 cannot separately detect the failure of the blue and red bands. It has also been found that optical amplification system 100 provides insufficient noise figure performance for certain applications.
What is needed are systems and methods for optical amplification that meet all of the objectives described above while permitting implementation within a small package.
Improved systems and methods for optical amplification of WDM signals are provided by virtue of one embodiment of the present invention. Multiple sub-bands of a WDM signal are amplified in a first common amplification stage. The sub-bands are separated from one another and amplified by independent parallel amplification stages. Each of the independent parallel amplification stages is equipped with a corresponding optical pump energy source. Furthermore, all of these optical pump energy sources together also provide the pump energy for the first common amplification stage. This architecture provides low noise figure and independent power regulation for each of the sub-bands while employing only Nxe2x88x921 pump energy sources for N amplification stages, thus greatly reducing space requirements and cost.
A first aspect of the present invention provides apparatus for amplifying a WDM signal. The apparatus includes: a first amplification stage that amplifies the WDM signal, an optical filter structure that separates the WDM signal into at least first and second sub-bands, a second amplification stage that amplifies the first sub-band and not other components of the WDM signal and that is pumped by a first pump, and a third amplification stage that amplifies the second sub-band and not other components of the WDM signal and that is pumped by a second pump. The first pump and the second pump contribute pump energy to the first amplification stage.
A second aspect of the present invention provides apparatus for amplifying a WDM signal. The apparatus includes N amplification stages. The N amplification stages include: a first amplification stage that amplifies a plurality of components of the WDM signal and Nxe2x88x921 amplification stages each associated with a sub-band of the WDM signal, each amplifying only the associated sub-band of the WDM signal after amplification by the first amplification stage. The apparatus further includes Nxe2x88x921 pumps where each provides pump energy to an associated one of the Nxe2x88x921 amplification stages. All Nxe2x88x921 pumps also contribute pump energy to the first amplification stage.